Jpn. Pat. Appln. KOKAI Publication No. 2002-160364 discloses a so-called shear-mode-type ink jet head which discharges ink from a plurality of nozzles. This type of ink jet head includes a piezoelectric ceramics plate, an ink chamber plate which is adhered to the surface of the piezoelectric ceramics plate, and a nozzle plate which is adhered in a manner to span between the end face of the piezoelectric ceramics plate and the end face of the ink chamber plate.
The piezoelectric ceramics plate includes a plurality of grooves and a plurality of partition walls. The grooves are arranged in line at intervals, and are continuously open to the surface and end face of the piezoelectric ceramics plate. The partition walls are interposed between neighboring ones of the grooves and isolate the grooves from one another.
The ink chamber plate closes the grooves of the piezoelectric ceramics plate from the direction of the surface of the piezoelectric ceramics plate. The nozzle plate closes the grooves of the piezoelectric ceramics plate from the direction of the end face of the piezoelectric ceramics plate. The inner surfaces of the grooves, the ink chamber plate and the nozzle plate cooperate and constitute a plurality of pressure chambers into which ink is supplied. Electrodes are formed on the surfaces of the partition walls which face each other, with the pressure chambers being interposed.
A plurality of nozzles are provided in the nozzle plate. The nozzles are obtained by applying laser machining using, for example, an excimer laser device, to the nozzle plate. The nozzles are minute holes on the order of microns, which penetrate the nozzle plate, and are open to the pressure chambers having the electrodes, respectively.
If a driving pulse is applied to the electrode, the partition walls facing each other, with the pressure chamber being interposed, deform and pressurize the ink that is supplied to the pressure chamber. The pressurized ink is discharged from the nozzle of the nozzle plate toward a recording medium on which printing is to be effected.
According to the ink jet head disclosed in the above-described KOKAI publication, a protection layer having electrical insulation properties is laid over each electrode. The protection layer has a two-layer structure comprising an inorganic insulation film and an organic insulation film. As the inorganic insulation film, use is made of an inorganic material such as silicon dioxide (SiO2). The inorganic insulation film covers the electrode and the inner surface of the groove. As the organic insulation film, use is made of an organic material such as polymonochloro-para-xylene. The organic insulation film is laid over the inorganic insulation film and covers the inorganic insulation film.
According to this protection layer, the inorganic insulation film has resistance to an organic solvent, and the organic insulation film has resistance to an inorganic chemical. Thus, even in the case where various kinds of inks having electrical conductivity are used, the electrical insulation between the ink and electrode can be ensured. Therefore, dissolution of the electrode can be prevented, and the discharge characteristic of ink can be improved.
According to the ink jet head disclosed in the above-described KOKAI publication, the protection layer is formed on the electrode after the piezoelectric ceramics plate and ink chamber plate are coupled. Thereafter, the nozzle plate is adhered in a manner to span between the end face of the piezoelectric ceramics plate and the end face of the ink chamber plate.
From the description of the above-described KOKAI publication, however, it cannot be understood whether the nozzle is formed in the nozzle plate before the nozzle plate is adhered to the piezoelectric ceramics plate, or the nozzle is formed in the nozzle plate after the nozzle plate is adhered to the piezoelectric ceramics plate.
In the case where the nozzle plate is adhered to the piezoelectric ceramics plate, it cannot be denied that an excess portion of an adhesive protrudes into the pressure chamber from between the piezoelectric ceramics plate and nozzle plate. At this time, if the nozzle is already formed in the nozzle plate, the excess portion of the adhesive protrudes towards the opening end of the nozzle that opens to the pressure chamber. Consequently, such a state occurs that the opening end of the nozzle is partially closed by the excess portion of the adhesive. If even a part of the opening end of the nozzle is closed, the flow of ink is disturbed when the ink is discharged. As a result, the discharge speed and discharge direction of ink become non-uniform, and the quality of print deteriorates.
On the other hand, in the case where the nozzle is formed in the nozzle plate after the nozzle plate is adhered to the piezoelectric ceramics plate, the adverse effect due to the adhesive can be avoided. Specifically, even if the excess portion of the adhesive protrudes into the pressure chamber, the excess portion of the adhesive is removed by a laser beam when the laser beam for forming the nozzle penetrates the nozzle plate and enters the pressure chamber. Thus, the excess portion of the adhesive does not adversely affect the flow of ink, and the degradation in print quality can be prevented.
The laser beam enters the pressure chamber immediately after penetrating the nozzle plate. In particular, in the case where the nozzle has a taper shape gradually widening toward the pressure chamber, the laser beam, which has penetrated the nozzle plate, is incident on the protection layer at an acute angle to the protection layer in the vicinity of the nozzle.
If the protection layer receives the laser beam, that part of the protection layer, which has been irradiated with the laser beam, is damaged. To be more specific, the laser beam for forming the nozzle in the nozzle plate has a wavelength less than visible light. Thus, if the organic insulation film of the protection layer, which is exposed to the pressure chamber, receives the laser beam, the organic insulation film evaporates and a damage hole opens in the organic insulation film.
As a result, the inorganic insulation film, which is covered with the organic insulation film, is exposed to the pressure chamber through the damage hole. In addition, depending on the thickness and refractive index of the inorganic insulation film, the laser beam may pass through the inorganic insulation film and may reach the electrode or piezoelectric ceramics plate.
If the protection layer is damaged by the laser beam, it is difficult to keep electrical insulation between the ink and electrode. As a result, for example, in the case where ink has electrical conductivity, that part of the electrode, which has received the laser beam, is dissolved, and the durability of the ink jet head lowers.
Furthermore, if the piezoelectric ceramics plate is damaged by the laser beam, the piezoelectric characteristics of the piezoelectric ceramics plate deteriorate. This leads to degradation in print quality of the ink jet head.